Blind attempt to fix LLVM dependency in the backend

Blind attempt to fix LLVM dependency in the backend

Commit ecaf7c5df5 removed gram.h from the backend's generated-headers
target. In LLVM builds, this leads to loss of dependency information
when generating .bc files. To fix, add a rule that mirrors ad-hoc .o
dependencies for .bc files as well.

Per cfbot (no buildfarm failures reported)
Analysis by Tom Lane and Andres Freund
Proposed fix by Andres Freund
Discussion: https://www.postgresql.org/message-id/20220914210427.y26tkagmxo5wwbvp%40awork3.anarazel.de